FHWA Seismic Retrofitting Seminar · – Seismic Retrofit Category A to D (SHL and SRC)Seismic...

FHWA Seismic Retrofitting gSeminar_______

Indianapolis, INp ,

October 19 20 2010October 19-20, 2010

AgendaAgenda

S i O iSeminar OverviewLesson 1 - Introduction to Seismic Retrofitting Manual

– PhilosophyPhilosophy– Methods for Screening– Evaluation Methods

Lesson 2 - Seismic Ground Motion Hazards and Geotechnical Hazards – Develop Response Spectrum

Di G t h i l H d i l di Li f ti– Discuss Geotechnical Hazards including Liquefaction Lesson 3 - Retrofitting Methods for SuperstructuresLesson 4 - Retrofitting Methods for SubstructuresLesson 4 Retrofitting Methods for SubstructuresLesson 5 - Retrofitting Methods for Abutments & FootingsQuestions and Answers Session and Final Exam

InstructorsInstructors

• Tom Saad, PE, Structural Engineer, FHWA Resource Center

J ti M PE G t h i l• Justice Maswoswe, PE, Geotechnical Engineer, FHWA Resource Center

D ll M PE St t l• Derrell Manceaux, PE, Structural Engineer, FHWA Resource Center

Seismic retrofitting manuals for ghighway bridges

1983: Seismic Retrofitting Guidelines for Highway Bridges (FHWA Report 83/007)g ( p / )

1995: Seismic Retrofitting Manual for Highway g g yBridges (FHWA Report 94-052)

2006: Seismic Retrofitting Manual for Highway Structures (FHWA Report …) Part 1: Bridges Part 2: Tunnels, walls, slopes, culverts..

FHWA Seismic Retrofitting ManualgPublication No. FHWA-HRT-06-032 (January 2006)

FHWA Seismic Retrofitting ManualgPublication No. FHWA-HRT-05-067 (August 2004)

FHWA Manual and AASHTO Specifications Terminology and Philosophy

• FHWA Seismic Retrofit Manual– Dual level ground motions (100 and 1000 yr. event)

Seismic Retrofit Category A to D (SHL and SRC)– Seismic Retrofit Category A to D (SHL and SRC)

• AASHTO LRFD Seismic Design Provision(2008)– 1000 yr design event– 1000 yr. design event– Seismic Zones 1-4

• AASHTO Seismic Design Guide SpecificationS O Se s c es g Gu de Spec cat o– 1000 yr. design event– Seismic Design Category A to D

• Standard Specifications– 500 yr. event – SPC A-D

State Earthquake Activity RankingState Earthquake Activity Ranking

C EQ F il M h iCommon EQ Failure Mechanisms

Unseating (most common) Column Shear Column Confinement Column Confinement Reinforcing Embedment and Laps Inadequate Foundation Capacity

UnseatingUnseating

Large displacements encountered during EQ can lead to superstructure unseating.p g

Unseatingg

Column ShearColumn Shear

Large shear forces encountered during EQ can lead to column shear failure.Q

Column ShearColumn Shear

Loss of Confinement

Large compressive stresses encounteredLarge compressive stresses encountered during EQ can lead to concrete crushing eventual loss of confinementeventual loss of confinement.

Loss of ConfinementLoss of Confinement

Is this a failure?Is this a failure?

Inadequate Reinforcing Embedment& Laps

Large forces encountered during EQ can lead to pull out of reinforcing.p g

I d t f d ti itInadequate foundation capacity

I d t f d ti itInadequate foundation capacity

Collapse due to liquefactionCollapse due to liquefaction

Learning OutcomesLearning OutcomesE l i h hil h f i i fi i i• Explain the philosophy for seismic retrofitting structures in accordance with the FHWA manual

• Develop a design response spectrum to determine the demand on p g p pthe structure

• Understand when liquefaction may be a consideration and discuss mitigation measuresmitigation measures

• Explain strategies for increasing capacity of existing structures• Explain strategies for decreasing demand on existing structuresp g g g• Establish State-wide policy and procedure for retrofitting structures

FHWA/NHI Bridge Design and Analysis Courses (www.nhi.fhwa.dot.gov)

NHI Course 130081: LRFD for Bridge Superstructures

NHI C 130082 LRFD f B id S b t t d ERSNHI Course 130082: LRFD for Bridge Substructures and ERS

NHI Course 130092: LRFR for Highway Bridges

NHI Course 130093: LRFD Seismic Analysis and Design of Bridges

NHI Course 130094: LRFD Seismic Analysis and Design of Tunnels, Walls and other Geotechnical Features

NHI Course 130095: LRFD: Design and Analysis of Skewed and Horizontally Curved Steel Bridges

Audience Expectations

Lesson 1Lesson 1 –Introduction to FHWA SeismicIntroduction to FHWA Seismic

Retrofitting Manual

IsBridgeYes BridgeExempt

?

N

Screen / prioritize

No

PassScreen / prioritize

FailPass

Evaluate

F ilReview

Pass

RetrofitNext bridge

FailReview

IsBridgeExemptExempt

?

Exempt bridges include those that are:

• Near end of service life (< 15 years remainingservice life)

• Temporary (less than a 15-year life)• Closed, but not crossing active roads, rail-lines, or

twaterways• In the lowest seismic zone

Performance-based retrofit__________________________________

Explicit attempt to satisfy public expectations ofExplicit attempt to satisfy public expectations of bridge performance for earthquakes ranging from small to large for example:small to large… for example:

EarthquakePerformance

Earthquake

Small Intermediate Large

No interruption √

√√

Limited access

Closed for

√ √√repairs √

Seismic Retrofit PhilosophySeismic Retrofit Philosophy

Small to Moderate Earthquakes:- resisted in the elastic rangeresisted in the elastic range- no significant structural damage

Large Earthquakes:- avoid collapse- damage rapidly detected & accessiblefor inspection and repair

Upper and lower level earthquakes

Lower Level earthquake (LL): 100-year return period100 year return period (50% probability of exceedance in 75 years)years)Upper Level earthquake (UL): 1000-year return period (7% probability of exceedance in 75(7% probability of exceedance in 75 years)

Performance-based retrofit

Application of performance-based design to bridge retrofittingdesign to bridge retrofitting two earthquake levels (Lower Level, Upper

Level)Level) two bridge types (standard, essential)

three service life categories (ASL 1 2 3) three service life categories (ASL 1,-2,-3) two performance levels (life safety,

operational)operational)

789

10Relative Effort

34567

PL1PL2

0123

Performance Level

HL1 HL2 HL3 HL4

PL0

Hazard LevelPL0 PL1 PL2

Seismic retrofit categories

Seismic Retrofit Categories, SRC, are used to recommend minimum levels of: screening evaluation retrofittingg

If these minima are satisfied, the required performance levels will berequired performance levels will be satisfied.

SRCs are similar to Seismic Design Categories (SDC) used in new design( ) g

Bridge Importance

AnticipatedService Life, ASL

Spectral Accelerations,

Ss and S1

Soil Factors, Fa and Fv

PERFORMANCE SEISMIC HAZARDPERFORMANCE LEVEL, PL

SEISMIC HAZARDLEVEL, SHL

SEISMIC RETROFITSEISMIC RETROFIT CATEGORY, SRC

Bridge importance

A bridge is essential if it satisfies one or more of the following:or more of the following: Provides access for emergency vehicles

and is required for secondary life safetyand is required for secondary life safety Would result in major social and / or

economic loss if collapsed or was closedeconomic loss if collapsed or was closed Required for security / defense

Crosses an essential route Crosses an essential routeAll other bridges are standard

Service life categories (ASL)

Se ice Life A ti i t d AgeService Life Category

Anticipated Service Life

Age (if not

rehabilitated) )

ASL 1 0 – 15 yrs 60 - 75 yrs

ASL 2 15 – 50 yrs 25 - 60 yrs

ASL 3 >50 years < 25 yrs

Bridge Importance



Ss and S1





Performance levels: PL0 and PL3

PL0: No minimum performance specifiedspecified.

PL3: Fully Operational: No collapse, no damage no interruption to trafficno damage, no interruption to traffic flow. No repair required.

Performance levels for bridge retrofitting

BRIDGE IMPORTANCE and SERVICE LIFE

EARTHQUAKE

and SERVICE LIFE

Standard Essential

ASL1 ASL2 ASL3 ASL1 ASL2 ASL3

Standard Essential

Lower Level

ASL1 ASL2 ASL3

PL0 PL3 PL3

ASL1 ASL2 ASL3

PL0 PL3 PL3PL0 PL3 PL3 PL0 PL3 PL3

Performance levels: PL1 and PL2

PL1: Life-safety: No collapse and life-safety preserved but damage will be severe particularly after UL event. Service is significantly disrupted. Bridge may need

l t ft UL treplacement after UL event.

PL2: Operational: No collapse, life-safety preserved, damage is minor, almost i di t f hi limmediate access for emergency vehicles, repairs feasible but with restrictions on traffic flowflow.

Performance levels for bridge retrofitting

BRIDGE IMPORTANCE and SERVICE LIFE

EARTHQUAKE

and SERVICE LIFE

Standard EssentialStandard Essential

ASL1 ASL2 ASL3 ASL1 ASL2 ASL3

Lower Level

ASL1 ASL2 ASL3

PL0 PL3 PL3

ASL1 ASL2 ASL3

PL0 PL3 PL3

Upper Level PL0 PL1 PL1 PL0 PL1 PL2

PL0 PL3 PL3 PL0 PL3 PL3

Upper Level PL0 PL1 PL1 PL0 PL1 PL2

Bridge Importance



Ss and S1





USGS hazard maps__________________________________

S i i h d l l I IVSeismic hazard levels: I - IV____________________________________

S ilSoilAcc

Bridge Importance



Ss and S1





Seismic retrofit category (SRC)Seismic retrofit category (SRC)__________________________________

HAZARD LEVEL

PERFORMANCE LEVEL

Upper Level EQ Lower Level EQHAZARD LEVEL

PL0: No min.

PL1:Life-safety

PL2: Operational

PL3:Operational

I A A B CI A A B C

II A B B C

III A B C CIII A B C C

IV A C D D

Minimum requirements q________________________________

ACTIONSEISMIC RETROFIT CATEGORY

ACTIONA B C D

Screening/Retrofitting

NRSeats, connections,

B + columns, walls

C + abutmentsRetrofitting liquefaction

walls, footings

abutments

EvaluationEvaluation Methods

NR A1/A2 B/C/D1/D2 C/D1/D2/E

Example:Data:Data:Essential bridge30-year service life remaining Bridge CityBridge City Dense soils (vs=1000 ft/sec)

Find:Seismic Retrofit Category, upper level earthquake.earthquake.

Example:

Service Life Category

Anticipated Service Life

Age (if not retrofitted)

ASL 1 0 – 15 yrs 60 - 75 yrs

ASL 2 15 – 50 yrs 25 - 60 yrs

ASL 3 >50 years < 25 yrs

Step 1: ASL2; site class C

BRIDGE IMPORTANCE d SERVICE LIFE

EARTHQUAKE

and SERVICE LIFE

St d d E ti lStandard Essential

ASL3ASL2ASL1ASL3ASL2ASL1

Lower Level PL3

ASL3

PL3PL0

ASL2ASL1

PL3PL3PL0

ASL3ASL2ASL1

Lower Level

Upper Level

PL3PL3PL0


PL3PL3PL0

pp

Step 2: Essential bridge; thereforePerformance criteria (UL) = PL1


Performance criteria (UL) = PL1

Step 3:S1=0 39g and SS=1 11gS1=0.39g and SS=1.11g

For site class C: Fv 1 4 and Fa 1 0Fv=1.4 and Fa=1.0

Fv*S1=0.55g and Fa*SS=1.11gd SHL IVand SHL = IV

Step 4: For PL1 SHL = IV andStep 4: For PL1, SHL = IV, and Seismic retrofit category is SRC= “C”

HAZARDPERFORMANCE LEVEL

HAZARD LEVEL PL0:

No min.PL1:

Life-safetyPL2:

OperationalNo min. Life safety Operational

I A A B

II A B B

III A B CIII A B C

IV A C D

Step 5: Minimum RequirementsStep 5: Minimum Requirements

ACTIONSEISMIC RETROFIT CATEGORY

ACTIONA B C D

Screening/Retrofitting

NRSeats, connections,

B + columns, walls

C + abutmentsRetrofitting liquefaction

walls, footings

abutments

EvaluationEvaluation Methods

NR A1/A2 B/C/D1/D2 C/D1/D2/E

Screening & Prioritization

Screen / prioritize

EvaluateEvaluate

Retrofit

Process for Lower Level earthquake F=Ma

Screening and prioritizationQ i k b d i f b i Quick screen based on comparison of basic earthquake load against wind and braking loads where earthquake load is taken asloads where earthquake load is taken as

F = FaSS.W = SDS.W If F < both F and F bridge passes If F < both Fwind and Fbraking, bridge passes If F > either Fwind or Fbraking, detailed

evaluation requiredevaluation required Prioritization for further evaluation based

on severity of shortfall in strengthon severity of shortfall in strength

Process for Lower Level earthquake (cont’d)

Detailed evaluation - Step 1 Calculate transverse and longitudinal

periods of bridge Calculate SaT and and SaL

Calculate FT = SaTW and FL = SaLW Calculate FT SaTW and FL SaLW If FT < Fwind and FL < Fbraking bridge passes,

otherwise go to Step 2otherwise go to Step 2


Detailed evaluation – Step 2 Calculate elastic, unfactored, strengths in

transverse and longitudinal directions, FcapTpand FcapL

If FT < FcapT and FL <FcapLbridge passes, T capT L capL g p ,otherwise retrofit is required for Lower Level earthquakeq


Retrofit strategy, approach, measuresStrategy: consider ‘do-nothing’ and ‘full-gy g

replacement’ options; identify relevant approaches (if more than one)

Approach: Decide most effective combination of techniques (measures) to satisfy performance requirement (PL3)

Measures: Devise retrofit measures… using conventional strength-based methodology.

Process for Upper Level EQProcess for Upper Level EQ

Process for Upper Level earthquake

Screening and prioritization

Detailed evaluationDetailed evaluation

Retrofit strategy and related approaches and measuresapproaches and measures

Screening and prioritizationPurpose is to screen an existingPurpose is to screen an existing inventory of bridges for seismic deficiencies and prioritize the inventorydeficiencies and prioritize the inventory for seismic retrofitting based on vulnerability hazard and non structuralvulnerability, hazard, and non-structural factorsS i h d d bScreening methods are expected to be quick and conservative; bridges that ‘fail’ are passed to a second level of screening i.e. ‘detailed evaluation’

Factors considered

Structural vulnerabilitySeismic and geotechnical hazardsOtherOther Importance Network redundancy Age and physical conditiong p y

Screening and prioritization

Three methods:Indices Method (FHWA 1995) Indices Method (FHWA 1995) Indices used for vulnerable components and

hazards and combined for single rating.hazards and combined for single rating. Expected Damage Method (new) Compares severity of damage includingCompares severity of damage including

economic loss. Seismic Risk Assessment Method (new)( ) uses network models and fragility functions

rank is based on direct and indirect losses, uses REDARS ftREDARS software

Evaluation of Performance

Screen / prioritize

EvaluateEvaluate

Retrofit

Methods of evaluation

In general, all evaluation methods i linvolve: Demand analysisy Capacity assessment Calculation of a capacity / demand ratio Calculation of a capacity / demand ratio

either for each critical component in a bridge or for each critical component in a bridge or for bridge as a complete system

Methods of evaluation (cont’d)Three categories six methods:Three categories, six methods:I. No demand analysis

1 M th d A ( it h k d f t d1.Method A (capacity checks made for seats and connections- 10% to 25% vertical reaction)

2 Method B (capacity checks made for seats2. Method B (capacity checks made for seatsconnections, columns, and footings- 25%

vertical reaction)II. Component C/D evaluation

3. Method C (elastic analysis: uniform load( ymethod, multimode spectral analysis;prescriptive rules given for calculation of component capacity)

Methods of evaluation (cont’d)

III. Structure C/D evaluation4. Method D1 (spectrum method: elastic4. Method D1 (spectrum method: elastic

analysis for demands, simplified models for calculation of capacity)

5. Method D2 (pushover method: elastic analysis for demands, nonlinear static analysis used for calculation of pieranalysis used for calculation of pier capacity)

6. Method E (nonlinear time history: analysis6. Method E (nonlinear time history: analysis for calculation of both demand and capacity)

Structural modeling

Load pathModeling recommendationsCombination of seismic forcesCombination of seismic forcesMember strength capacitiesMember deformation capacities

Load path

Identify clear load path for lateral loads: D k l b d t ( t d ) Deck slab and connectors (studs)

Cross frames (diaphragms) Longitudinal beams (girders) Bearings and anchorages Pier (cap beam, columns, walls) Abutments and foundations (back wall, ( ,

footing, piles) Soils

Structural modeling recommendations

Distribution of massDistribution of stiffness and strengthDampingDampingIn-span Hinges Substructures SuperstructuresSuperstructures

Combination of seismic forces

Loading in 2- or 3-orthogonal directions: 100-40% Rule

Member strength capacities

Flexural and shear strength of i f d t l d breinforced concrete columns and beams

Design vs. Actual flexural strengthg g Design vs. Actual shear strength Flexural overstrength Flexural overstrength Flexural strength of columns with lap-

splices in plastic hinge zonessplices in plastic hinge zones

Member deformation capacities –Chapter 7

Plastic curvature & hinge rotationsD f ti b d li it t tDeformation-based limit states Compression failure of confined and

unconfined concrete Buckling longitudinal bars Tensile fracture longitudinal bars Low-cycle fatigue longitudinal barsy g g Failure in lap-splice zone

Retrofit Strategies, Approaches and MeasuresApproaches, and Measures

Screen / prioritize

EvaluateEvaluate

Retrofit

Retrofit strategies, approaches, and measures

Retrofit Measure: a device or t h i h t i ltechnique such as a restrainer, column jacket, stone column…Retrofit Approach: One or more measures used together to achieve anmeasures used together to achieve an improvement in performance such as t th i i t i dstrengthening using restrainers and

jackets…

Retrofit strategies, approaches and measures (cont’d)

Retrofit Strategy (one of the f ll i )following): One or more approaches used together to pp g

achieve desired level of improvement in performance such as strengthening and p g gsite remediation.

Partial or full replacement Partial or full replacement Do-nothing (retrofitting not justified)

Retrofit approaches

Approaches: one or more measures to achieve:ac e e Strengthening Displacement capacity enhancement Displacement capacity enhancement Force limitation

Response modification Response modification Site remediation

P ti l l t Partial replacement Damage acceptance or control

Retrofit measures

Superstructure measures:Restrainers Restrainers

Seat width extensions, catcher blocksC ti i l Continuous simple spans

Bearing side-bar restraints, shear keys, tstoppers

Isolation bearings and energy dissipators, i l di d til d di hincluding ductile-end-diaphragms

Retrofit measures (cont’d)

Substructure measures C l j k ti i t l fib Column jacketing, using steel, fiber composites, or concrete shells

Infill walls Column replacementsp

Retrofit measures for foundations and hazardous sites

Retrofit Measures for Abutments, Footings and Foundations Hazardous sites including g near active faults unstable slopes p liquefiable sites.

Summary

SummaryP f b d hil h ( th d l )Performance-based philosophy (methodology): two earthquake levels (Lower Level, Upper Level) two bridge types (standard, essential) three service life categories (ASL1,-2,-3) two performance levels (life safety, operational)

Three-stage process for each earthquake level: screening,screening, evaluation, and retrofit retrofit

Summary (cont’d)

Seismic Retrofit Categories, SRC, are used to recommend minimum levels ofrecommend minimum levels of screening evaluation, and evaluation, and retrofitting

SRCs are equivalent to Seismic DesignSRCs are equivalent to Seismic Design Categories (SDC) used in new designSRC b d h d l l d d i dSRCs are based on hazard level and desired performance level

Summary (cont’d)

Three screening methodsSix evaluation methodsSix evaluation methodsRetrofit phase divided into three steps Decide strategy Select approach pp Design and install component retrofit

measures

Summary (cont’d)

Step 1. For Lower Level earthquake: Screen, evaluate, retrofit (controlled by service

loads such as wind and braking )loads such as wind and braking…)

Step 2 For Upper Level earthquake:Step 2. For Upper Level earthquake: Calculate seismic retrofit category Screen and prioritizep

For bridges that do not pass screen: Conduct detailed analysis for demand and

l t itevaluate capacity Decide retrofit strategy, select approach, and

design & install retrofit measuresg

What questions do you have?What questions do you have?

FHWA Seismic Retrofitting Seminar · – Seismic Retrofit Category A to D (SHL and SRC)Seismic...

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